Magnetic drive couplings are known in which input and output rotors are provided with respective sets of permanent magnets, and a set of ferromagnetic pole pieces is arranged between the ‘rotors to modulate the magnetic field, such that the output rotor is driven at a different speed to the input rotor, providing a geared coupling. In a variation of such systems, the pole pieces are mounted on one of the rotors and one set of permanent magnets is fixed.